frankly, if it were my dog, i wouldn't prolong her life.
dogs don't think the way we do - "life above all". to them, it's quality of life that matters. in her mind, she's still a young, fit pup who wants to romp and play and do all the things she is accustomed to doing. if she tries but her body is failing her, this is totally against nature and is psychologically traumatizing to her.
if you want to do what's best for you, pay the money for the meds and make her live as long as you can - but you're only putting off the inevitable.
if you want to do what's best for your dog, give her a dignified ending to a cherished life.